An offshore platform is generally composed of two sections: 1) a substructure such as a jacket for a fixed platform, and 2) a superstructure such as a deck to be installed on the top of a substructure.
A deepwater substructure, deeper than 60 meters (about 200 ft) in water depth, of a fixed platform is normally fabricated as a single unit with battered leg onshore in a horizontal orientation and then skidded onto a transport vessel or a launch vessel, towed to the installation site in a horizontal orientation, launched or lifted off from the vessel, and placed at the seabed before upending/ballasting of the jacket to a vertical position. Finally, foundation piles are driven to fix the jacket with the seabed by grouting or welding.
A shallow water substructure, less than 60 meters (about 200 ft) in water depth, of a fixed platform is normally fabricated as a single unit with vertical legs onshore in a vertical orientation and then skidded onto a transport vessel or a semi-submersible vessel, towed to the installation site in a vertical orientation, lifted off the transport vessel deck, or lifted off the semi-submersible vessel deck when it is submerged to a design draft, and placed at the seabed in a vertical orientation throughout the installation operations. Finally, foundation piles are driven to fix the jacket with the seabed by welding between foundation piles and jacket leg tops.
For a typical shallow water jacket configuration, especially a large sized one, it is very difficult to gain sufficient net buoyancy. Therefore, a large crane installation with a lifting capacity larger than the weight of the jacket has to be utilized to lift the jacket as a whole off the transport vessel deck, or the semi-submersible vessel deck, and to place the jacket at the seabed.
In recent years, shallow water jackets get heavier and heavier because the associated deck weights also get heavier and heavier. In many cases, the jacket weights exceed the lifting capacity of available crane vessel(s) and alternative jacket installation methods have to be considered. One common alternative method is to launch the jacket. If the launching method is adopted, the jacket orientation on the transport vessel is usually changed to a horizontal orientation. In addition, it has to face two common challenges:
1. The jacket has to be a self afloat structure with necessary reserve buoyancy (usually >12%, defined as (submerged buoyancy−total weight)/submerged buoyancy %). In order to satisfy this requirement, a large number of steel-made buoyancy tanks have to be installed and connected to the jacket and to make this jacket even heavier. Ballast tanks and flooding/venting systems have to be designed in order to lower the jacket to seabed through ballasting operations. These buoyancy tanks have to be removed after the installation and transported back onshore at a considerable cost. Other costs include fabrication and installation of the buoyancy tanks and the design and fabrication of the ballast tanks. Another issue is that the weight of steel makes the steel-made buoyancy inefficient to produce net buoyancy and very costly for each ton of net buoyancy. For example, one ton steel used for making buoyancy tanks could typically produce 3-ton buoyancy. If deducting the steel weight, each ton of steel could produce only 2-ton net buoyancy. Adding other costs such as design, fabrication, flooding/venting system, welding to a jacket, offshore cutting to remove from the jacket, lifting and the use of a transport vessel for returning the tanks back, the total cost of using buoyancy tanks could be very high.
2. Due to the shallow water at the installation site, a launched jacket could easily hit the seabed during the launch operation. In such cases, the jacket is usually towed to a deeper water location, launched and wet towed from the launching site to the installation site. If the launching site is far away from the installation site, the cost associated with the wet tow could be high.
A heavy shallow water jacket could be launched in a vertical orientation. However, it would require larger reserve buoyancy (>20%) and the attached buoyancy tanks have to be placed at very low position, to pick up buoyancy immediately after the launch, which would impose extra difficulty for removing these buoyancy tanks because they would be all submerged after the launch.
Therefore there is a need for a shallow water jacket installation method that is more efficient in producing net buoyancy and cost effective.